In optical communications, FIG. 1A is a generic illustration of adjacent data channels in a fixed grid optical spectrum including guard bands between data channels. In fixed grid optical systems individual channels have equal spectral width as illustrated, whereas in gridless (also known as flex-grid) optical systems individual data channels have differing spectral widths as illustrated in FIG. 1B. When multiple data channels travel together sharing the same optical path from one source to the same destination, the channels are “packed” into one Network Media Channel (NMC) as defined in ITU-T G.870.
In practice, data channel width considerations account for electromagnetic signal propagation properties. Before being transmitted on an optical link, signals are shaped to minimize inter-symbol interference. An example of typical pulse shaping employed is referred to as “raised-cosine” with parameter β defining an amount of excess bandwidth employed. The frequency domain equation is as follows:
                                          X            rx                    ⁡                      (            f            )                          =                  {                                                                      T                                                                      0                    ≤                                                                f                                                              ≤                                                                  1                        -                        β                                                                    2                        ⁢                        T                                                                                                                                                                                    T                      2                                        ⁢                                          {                                              1                        +                                                  cos                          ⁡                                                      [                                                                                                                            π                                  ⁢                                                                                                                                          ⁢                                  T                                                                β                                                            ⁢                                                              (                                                                                                                                          f                                                                                                        -                                                                                                            1                                      -                                      β                                                                                                              2                                      ⁢                                      T                                                                                                                                      )                                                                                      ]                                                                                              }                                                                                                                                                          1                        -                        β                                                                    2                        ⁢                        T                                                              ≤                                                                f                                                              ≤                                                                  1                        +                        β                                                                    2                        ⁢                                                                                                  ⁢                        T                                                                                                                                          0                                                                                                                  f                                                              >                                                                  1                        +                        β                                                                    2                        ⁢                        T                                                                                                                  .                                              (        1        )            
FIG. 1C illustrates the corresponding resulting channel spectrum for different β. For certainty, pulse shaping is not limited to raised-cosine.
The excess bandwidth such as defined by β is employed by the transmitter to provide clock information along with the data in the channel. The receiver uses the information in the excess bandwidth to extract a data clock signal and to estimate clock jitter.
Regardless of use of channels in a fixed grid deployment or in a gridless deployment there is a need to increase bandwidth utilization in a fiber. In gridless systems increased bandwidth utilization seeks to increase channel density. In fixed grid systems bandwidth utilization can be increased by increasing channel width.
As channel width/density increases, data transmission in a channel is subject to interchannel interference from neighboring channels deployed over the same fiber. FIG. 1D illustrates a typical received signal power distribution of a data channel used as a reference for description purposes herein. While shape similarities with signal shaping at the transmitter illustrated in FIG. 1C are apparent, away from the data channel as received at the receiver, signal energy leaks into neighbor channels. The energy leaks primarily affect the area of excess bandwidth of a particular adjacent neighbor channel. Trailing signal power away from the reference channel distribution contributes to increased background noise in neighbor channels to the reference channel.
Increasing channel width into the guard bands causes the distribution of leaked channel power to overlap in the guard bands as is more apparent from FIG. 1A. As excess bandwidth in the guard bands is used for clock recovery at the receiver, spectral energy leaked from adjacent channels affects the integrity of the extracted data clock which manifests itself as an increase in clock jitter.
Typically, as illustrated in FIG. 1B guard bands are reserved to provide channel separation to limit interchannel interference. The reservation of the guard bands may be based on agreement between operators to provide interoperability.
From a data transport point of view, the guard bands illustrated in FIG. 1B are dead-bands on the edges of each NMC. Guard bands can take up to 6.25 GHz roll-off and as such take up valuable spectrum.